Inhomogeneous broadening and pressure shifts of the optical spectra in organic glasses at low temperatures

Inhomogeneous bandwidths were measured at 6 K in glassy solutions of poly(m ethyl methacrylate) and ethanol for chromophores with comparatively weak el ectron-phonon coupling: polymethine dyes, polycyclic hydrocarbons and tetra pyrrolic compounds. The contributions of dispersive attraction, the interna l electric field effects and the repulsive forces to the band broadening ar e considered. Spectral holes were burned in the inhomogeneous S-1 --> S-0 a bsorption bands and the pressure shift coefficients dv/dP were determined u sing gaseous He as pressure transmitter. Pressure shift coefficients show a linear dependence on hole-burning frequency. The slope of this dependence generally differs from the value of 2 beta(T) (beta(T) is the isothermal co mpressibility of the matrix), predicted from the dispersive solvent shift e quation. The slopes smaller than 2 beta(T) are accounted for in terms of ot her solvent shift mechanisms, such as linear and quadratic Stark effects in the cavity field. The slopes larger than 2 beta(T) are ascribed to repulsi ve interactions. (C) 2000 Elsevier Science B.V. All rights reserved.